Make image chooser "Select format" fields translatable

- Add explanatory comment
- Make form labels translatable

wagtail/images/forms.py

@@ -95,11 +95,12 @@ class ImageInsertionForm(forms.Form):
     into a rich text area
     """
     format = forms.ChoiceField(
+        label=_("Format"),
         choices=[(format.name, format.label) for format in get_image_formats()],
         widget=forms.RadioSelect
     )
-    image_is_decorative = forms.BooleanField(required=False)
-    alt_text = forms.CharField(required=False)
+    image_is_decorative = forms.BooleanField(required=False, label=_("Image is decorative"))
+    alt_text = forms.CharField(required=False, label=_("Alt text"))
 
     def clean_alt_text(self):
         alt_text = self.cleaned_data['alt_text']

wagtail/images/views/chooser.py

@@ -241,6 +241,8 @@ def chooser_select_format(request, image_id):
     else:
         initial = {'alt_text': image.default_alt_text}
         initial.update(request.GET.dict())
+        # If you edit an existing image, and there is no alt text, ensure that
+        # "image is decorative" is ticked when you open the form
         initial['image_is_decorative'] = initial['alt_text'] == ''
         form = ImageInsertionForm(initial=initial, prefix='image-chooser-insertion')